1. Field of the Invention
This invention relates to a vehicle speed control system, more particularly to a vehicle speed control system equipped with a cruise controller and a CVT (Continuously Variable Transmission).
2. Description of the Related Art
Japanese Laid-open Patent Application No. Hei 10 (1998)-29448 teaches a cruise controller for a vehicle equipped with a CVT. This cruise controller has a followup operation to follow up another vehicle running ahead of the subject vehicle. In this prior art system, when the followup operation is in progress, the ratio (speed ratio) of the CVT is controlled at a fixed ratio and it is determined whether required torque is available. If the required torque is not available, the air/fuel ratio to be supplied to the engine is enriched, thereby reducing the engine speed fluctuation and preventing wandering of the engine speed from happening.
Vehicles equipped with a CVT can act quickly in response to the change of throttle opening operated by the vehicle operator, thereby enhancing the acceleration performance. However, this is disadvantageous when the vehicle conducts cruise control which requires no rapid acceleration. Specifically, when the vehicle under cruise control travels on a road whose grade varies, if the throttle actuator operates to open the throttle valve temporarily so as to maintain a desired vehicle speed, the CVT controller responds too quickly, thereby resulting in the engine speed changing and sometimes making the vehicle operator feel uneasy.
This will be explained with reference to FIG. 9. Assume that the gradient of the road (on which the vehicle climbs) changes as illustrated in the figure by solid line. As the vehicle advances the upsloping road, the vehicle speed drops and as a result, the cruise controller operates the throttle actuator to open the throttle valve so as to raise the vehicle speed toward a desired speed. Parallel to this, the CVT controller controls the ratio (speed ratio) of the CVT to the low side.
As the vehicle speed approaches the desired speed, the cruise controller returns the throttle valve to the closing direction and in response thereto, the CVT controller controls the ratio in the high side to a value, which deviates in the low side by an amount .alpha., for example. However, the CVT controller responds to the throttle actuator operation to open the throttle valve so as to correct the vehicle speed drop, it will further control the ratio to the low side by an additional amount .beta., for example. In the upslope change illustrated, it suffices if the ratio is controlled only by the amount .alpha.. The unnecessary amount .beta. will raise the engine speed, making the vehicle operator feel uneasy.
Although the earlier prior art mentioned above teaches cruise controller for vehicles equipped with a CVT, it does not go beyond simple measures such as fixing the speed ratio when the followup operation is in progress and is therefore incapable of offering a solution to the issues discussed above.